Aluminum base alloy



United States Patent minum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No-Drawing. Application January 13, 1958 Serial No. 708,351

5 Claims. (Cl. 75-142) This invention relates to high strength aluminum base alloys and more particularly to certain compositions which retain a relatively high strength when heated to and heldat elevated temperatures.-

The properties and performance of aluminum and aluminum base alloy products at room temperature are well-known, and it is also known that castings and forgings of certain alloys have served well in reciprocating-type internal combustion engines for such parts as pistons, cylinder heads and cylinder barrels when the barrels are provided with a ferrous metal liner. With the development of high. speed aircraft and other ma: chines that are exposed to or develop high temperatures, there has'been a demand for aluminum base alloys which will have a higher strength than the commercial compositions heretofore available. There has been a particular need for high strength alloys which can withstand aerodynamic heating for short periods of time. In addition to the requirement for higher strength there has been a need for alloys which have a greater resistance to creep at elevated temperatures than previous compositions. Such alloys should also be amenable to being rolled or otherwise converted into articles havingv relatively thinsections.

It is, therefore, an object of this invention to provide an aluminum. base alloy that possesses a high strength and resistance creep at elevated temperatures and that can still be readily worked.

Another object is to provide an aluminum base alloy which in the solution heat treated and age hardened condition 'retainsits room temperature properties to a high degree whenheated to. elevated temperatures below about 400 F. t Still another objectjs, to provide an aluminum base alloy which in addition to possessing a high strength and resistance to creep at elevated temperatures also has a higher .modulus of elasticity than is exhibited by previous commercial aluminum base alloys.

These and other objects and advantages are realized in aluminumflbasealloys consisting essentially of aluminunj',' from3 to.9%, copper, 0.15 to 1.0% manganese, 0.02 10.5 cadmium, 0.2 to 3% lithium, 0.05 to 2.0% magnesium and, the usual impurities associated with aluminum where the alloy has been rolled, forged, ex truded, pressed or.otherwise worked and then heat treated.

To attain the maximum strength at elevated temperatures, the wrought alloy should receive a solution heat treatment followed byage hardening above room temperature, In this conditionanalloy consisting essentially of aluminum, 4.5 %c opper, 0.5% manganese, 0.3% magnesium, 0.15% cadmium and 1% lithium will develop a'itensile strength, at 300" F. of-74,000 p.s.i., a yield strength of 71,000 psi, and an elongation of 12% after an exposure of "100 hours. By comparison a commercial solution heat treated and precipitation hardened aluminum base. alloy nominally consisting of aluminum, 4.5% copper, 0.6% manganese and 1.5% magnesium has a tensile strength at 300 F. of 58,000 p.s.i., a yield strength "2,915,391 Patented Dec. 1, 1959 ice of 46,000 p.s.i. and an elongation of 19%. In any case, the wrought alloy of this invention in the heat treated and precipitation condition, possesses a higher tensile .andyield strength than the same alloy without cadmium, lithium and magnesium. Furthermore, it has a typical resistance to creep at 300 F. of 0.005 in./ in. for a period of 100 hours under a stress of 58,000 p.s.i. At room temperature the new alloy has a modulus of elasticity of 11.2)(10 p.s.i. as compared to only 10.4 10 p.s.i. for similar aluminum base alloys containing copper as the chief added alloy component.

The alloy can be hot and cold Worked by conventional practices in a manner similar to other high strength aluminum alloys.

To achieve the desired properties the several elements must be used in the indicated proportions. If less than 3% copper is present, the alloy will not have the re quired strength while an alloy containing more than 9% copper is not readily rolled or otherwise worked. In the preferred practice of the invention the copper content is kept between 4.5 and 6%.

The manganese component is also essential to developing high strength at elevated temperatures and should be present in amounts of 0.15 to 1.0%. Larger amounts interfere with working of the alloy.

The cadmium, lithium and magnesium components appear to interact in a-manner not understood to increase the strength above that provided by the copper and manganes'e components alone. Although the melting points ofcadmium and lithium are much lower than those of aluminum and these elements therefore might be expected to be detrimental to a high strength at elevated temperatures, it has, nevertheless, been found that they, in combination with magnesium, have a beneficial effect uponthe strength of the aluminumcopper-manganese base composition. Furthermore, the susceptibility to oxidation of these elements does not appear to detract from the performance of the alloy at elevated temperatures.

The amount of cadmium required, 0.02 to 0.5%, is relatively small, but, nevertheless, essential in the total composition. Larger amounts are undesirable because of the possible presence of free or uncornbined cadmium which is detrimental to the working characteristics of the alloy. The lithium content is also relatively small and must be within the range of 0.2 to 3%. Less than 0.2% does not afford any significant improvement in thealloy while more than 3% offers no further increase in'properties at room temperature or at 300 F. The third element of the group, magnesium, must be present in an amount of at least 0.05% to obtain the desired strength but more than 2% is unnecessary and makes working difiicult. It is preferred, however, to employ 0.15 to 0.6 of this element to obtain optimum properties;

Although cadmium is preferred as the element to be i used in combination with lithium and magnesium, it may used, but still these properties are superior to those found in similar compositions devoid of lithium, magnesium andIthe equivalent of cadmium.

Undersome conditions of casting it may be desirable to add certain elements to the alloy to refine the grain size in the ingot. For this purpose at least one element of the group composed of boron, titanium, vanadium and zirconium may be employed in amounts of 0.002 to 0.05% boron, 0.01 to 0.25% titanium, 0.02 to 0.3% zirconium and 0.01 to 0.1% vanadium, the total amount of all of these elements not exceeding 0.5%.

The silicon impurity content of the alloy may be as high as 0.6% without adverse effect upon the strength of the alloy at elevated temperatures, but it is preferred to maintain a maximum of 0.2%. The iron impurity should not exceed about 0.6%.

To attain the high strength properties at elevated temperatures the alloy should receive a solution heat treatment of l to 12 hours at 920 to 980 F., be quenched and then be aged for 5 to 200 hours at 275 to 350 F. The solution temperature used is governed by solidus temperatures for the specific alloys being treated. The beneficial effect of the thermal treatment upon the strength at elevated temperatures is surprising since it would be anticipated that further exposure to temperature at which age hardening occurs would cause overaging with resultant loss in strength.

If desired, the aged alloy product may be cold worked to further increase the tensile and yield strength and hardness.

The alloy possesses the remarkable property of undergoing a relatively small decrease in strength when heated to a temperature on the order of 300 F. as compared to the properties at room temperature. Furthermore, the alloy can be repeatedly heated to such a temperature and cooled to room temperature without any substantial detrimental effect upon the strength at elevated temperatures. This behavior of the alloy is particularly valuable for skin coverings of high speed aircraft which are subject to aerodynamic heating. In such service the alloy may be quickly heated and cooled and, hence, it is imperative that the structural portions of the aircraft retain their high strength properties during repeated flights.

The improvement is tensile and yield strength and resistance to creep exhibited by alloys of this invention is illustrated in the following comparison between them and a commercial alloy composed essentially of aluminum, copper, manganese and titanium which has been proposed for use at elevated temperatures. The latter composition may be regarded as representative of alloys which do not contain cadmium, lithium and magnesium. The composition of the alloys tested is given in Table I below, the balance of the alloy in each case consisting of aluminum.

TABLE I Composition of alloys Per- Per- Per- Per- Per- Per- Per- Percent cent cent cent cent cent cent cent Cu Mn Li Cd Mg Ti Fe The alloys were melted, cast and forged to diameter rods which were cut into lengths suitable for later machining into standard tensile test bars and creep test specimens. The forged bars of alloy A were solution heat treated two hours at 1000 F., quenched in water and aged 12 hours at 375 F. The bars of the remaining alloys with the exception of alloy D were solution heat treated two hours at 960 F., quenched in water and aged 12 hours at 320 F. Alloy D was heat treated at 940 F. The difference in solution heat treatment was dictated by the difference in composition, a lower temperature being employed to treat the improved alloys in order to avoid incipient fusion of any low melting constituents,

TABLE II Tensile properties at room and elevated temperatures ROOM TEMPERATURE Tensile Yield Elonga- Alloy Strength Strength, tion, perp.s.i. p.s.l. cent Hours Alloy Tensile Yield Elong., Tensile Yield Elong.,' Strength, Strength, Percent Strength, Strength, Percent p.s.l. p.s p.s.l. p.s.l.

AT 400 F.

% Hour 100 Hours Alloy Tensile Yield Elong., Tensile Yield El0ng., Strength, Strength, Percent Strength, Strength, Percent p.s.l. p.s.l. p.s.l. p.s.l.

It is apparent that alloys of this invention not only had much higher room temperature properties but maintained a higher level of strength at 300 F. and 400 F. than the aluminum-copper-manganese base alloy with the exception of alloy E exposed 100 hours at 400 F. Alloy E, it should be noted, contained only 0.75% lithium whereas the other alloys contained 1% or more of that element. In particular it is to be seen that the tensile strength of the improved alloys was reduced less than 15% by heating to 300 F. and less than 30% upon heating to 400 F. This is a relatively small change as compared to many commercial aluminum base alloys. It is also noteworthy that the copper contentv of alloys B, C, D, E and F was lower than in alloy A and yet a higher strength was attained thus further emphasizing the benefit gained from the presence of lithium, cadmium and magnesium.

For the creep tests, specimens of alloys A, B, C and D were employed that had received the solution heat treatment and aging referred to above. Alloy A was tested at 400 F. but alloys B, C and D were tested at both 300 and 400 F. The creep' test consisted of stress ing the specimens sufficiently to cause a creep of 0.001, 0.002, 0.005 and 0.01 in./in. and final rupture. The periods of time at which the stipulated amount of creep occurred and the stresses which produced the creep are given in Table III. The stress values are the average for alloys B, C and D; the values for the individual alloys being too close together to indicate any significant difference between them.

TABLE III Average creep and stress rupture values at 300 and 400 F.

ALLOYS B, O, D AT 300 F.

STRESS (P.S.I.) REQUIRED TO CAUSE VARIOUS AMOUNTS OF TOTAL CREEP AT 400 F.-ALLOY A STRESS (P.S.I.) REQUIRED FOR VARIOUS AMOUNTS OF TOTAL OREEP Time under 0001 0.002 0.005 0.01 Stress for Stress, Hour m./i1 1., in./in., in./i1 1., in./i1 i., Rupture,

p.s.i. p.s.i. p.s.i. p.s.1. p.s.i.

ALLOYS B, 0 AND D Time under 0.001 0002 0.005 0.01 Stress for Stress, Hour 1n./in., 1n./m., in./in., in.lin., Rupture,

p.s.i. p.s.i. p.s.i. p.s.i. p.s.1.

It is apparent from the test results that alloys B, C and D had a much higher resistance to creep and stress rupture than alloy A at 400 F. It is also evident that although the values at 300 F. are higher than those at 400 F. an advantage is still gained from the addition of lithium, cadmium and magnesium to the aluminum-copper-manganese base.

Having thus described my invention and certain embodiments thereof I claim:

1. A wrought aluminum base alloy consisting essentially of aluminum, 3 to 9% copper, 0.15 to 1.0% manganese, 0.02 to 0.5% cadmium, 0.2 to 3% lithium and 0.05 to 2.0% magnesium, said alloy when solution heat treated at 920 to 980 F. and aged at 275 to 350 F. being characterized by a higher tensile and yield strength at 300 to 400 F. than the same alloy devoid of cadmium, lithium and magnesium.

2. An alloy according to claim 1 having a copper content of 4.5 to 6%.

3. An alloy according to claim 1, having a magnesium content of 0.15 to 0.6% magnesium.

4. A wrought aluminum base alloy consisting of aluminum, 3 to 9% copper, 0.15 to 1.0% manganese, 0.02 to 0.5% cadmium, 0.2 to 3% lithium, 0.05 to 2.0% magnesium and at least one element selected from the group composed of 0.002 to 0.05% boron, 0.01 to 0.25% titanium, 0.02 to 0.3% zirconium and 0.01 to 0.1% vanadium, said alloy when solution heat treated at 920 to 980 F. and aged at 275 to 350 F. being characterized by a higher tensile and yield strength at 300 to 400 F. than the same alloy devoid of cadmium, lithius and magnesium.

5. A wrought aluminum base alloy consisting essentially of aluminum, 3 to 9% copper, 0.15 to 1.0% manganese, 0.2 to 3% lithium, 0.05 to 2% magnesium and References Cited in the file of this patent UNITED STATES PATENTS 1,750,700 Berthelemy Mar. 18, 1930 2,219,095 Schuttler Oct. 22, 1940 2,381,219 Le Baron Aug. 7, 1945 2,579,369 Dawe Dec. 18, 1951 2,784,126 Criner Mar. 5, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,915,391 December 1, 1959' Charles B, Criner It is hereby certified thet error appears in the -printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 38, for improvement is" read improvement in column 4, Table II, in the group headed "AT 400 fifth column thereof, under the heading "Tensile Strength, p.,s.i.", fourth line, for "45,303" read 45,300 column 6, line 24, for "lithius" read m lithium Signed and sealed this 17th "day of Ma 1960 (SEAL) Attest:

KARL H. AXLINE ROBERT c. WATSON Attesting Officer Commissioner of Patents 

1. A WROUGHT ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM 3 TO 9% COPPER, 0.15 TO 1.0% MANGANESE, 0.02 TO 0.5% CADMIUM, 0.2 TO 3% LITHIUM AND 0.05 TO 2.0% MAGNESIUM, SAID ALLOY WHEN SOLUTION HEATED TREATED AT 920 TO 980*F. AND AGE AT 275 TO 350* F. BEING CHARACTERIZED BY A HIGHER TENSILE AND YIELD STRENGTH AT 300 TO 400*F. THAN THE SAME ALLOY DEVOID OF CADMIUM, LITHIUM AND MAGNESIUM. 